The histamine-3 (H3) receptor is one of four histamine receptor subtypes (H1-H4), all of which are members of the G-protein-coupled receptor (GPCR) superfamily. The H3 receptor is predominantly expressed in the central nervous system. In the brain, it is located in regions associated with learning and memory such as the cerebral cortex, hippocampus and striatum.
The H3 receptor acts as both an auto- and hetero-receptor to regulate the release of histamine and other neurotransmitters. Within the cortex, the H3 receptor appears to directly modify GABA release from cortical interneurons. Antagonism of the H3 receptor produces a decrease in GABA release and disinhibition of the cortical cholinergic system, resulting in increased acetylcholine levels (Bacciottini, L. et al, Behavioral Brain Research, 124, 2001, 183-194). In addition to direct regulation of cholinergic neurotransmission, the H3 receptor has been shown to modulate the release of dopamine, serotonin and norepinephrine (Leurs, R., et al, Trends in Pharmacological Sciences, 19, 1998, 177-183). Thus, H3 receptor blockade is able to elevate concentrations of a number of neurotransmitters, including: histamine, acetylcholine, dopamine, serotonin, norepinephrine, and glutamate, and thus offers a means for targeting cognitive processes, which often rely on the integration of multiple neurotransmitter systems.
H3 agonists have been reported to impair memory in various tasks, such as object recognition, passive avoidance (Blandina, P., et al, British Journal of Pharmacology, 119(8), 1996, 1656-1664) and social olfactory memory (Prast, H., et al, 734, 1996, 316-318), whereas H3 antagonists have been reported to rescue impairments produced pharmacologically or genetically. Miyazaki, S., et al, Life Sciences, 61, 1997, 355-361; Meguro, K., et al, Pharmacology, Biochemistry and Behavior, 50, 1995, 321-325; Fox, G. B., et. al, Behavioral Brain Research, 131, 2002, 151-161; and Komater, V. A., et al, Psychopharmacology, 167, 2003, 363-372.
H3 receptors are targets for the control of arousal and vigilance as well as for the treatment of sleep disorders because they colocalize with histaminergic neurons in brain regions that regulate the sleep-wake cycle and they modulate histamine release and levels in the CNS. Passani et al. Trends Pharmacol. Sci. 25, 618-25, 2004. The administration of selective H3 receptor agonists, such as R-α-methylhistamine, increases sleep time and slow wave sleep in cats and rodents and produces sedation in the guinea pig, whereas H3 antagonists such as thioperamide increase wakefulness in cats and rats and decrease slow wave sleep and REM sleep in rats. Monti et al. Eur. J. Pharmacol. 205, 283-287, 1991 and Esbenshade et al. Molecular Interventions 6:77-88, 2006.
Studies on memory consolidation and spatial memory impairments, which are particularly prevelant in AD and dementia, have revealed that the H3 antagonist thioperamide improves recall in a mouse model of premature senescence as well as in spontaneously hypertensive rat pups, and also prevents scopolamine-induced amnesia. Meguro et al. Pharmacol. Biochem. Behav. 50, 321-325, 1995 and Hancock et al. Expert Opin. Investig. Drugs 13, 1237-1248, 2004. Further, H3 receptor knockout mice are insensitive to the effects of scopolamine in an inhibitory avoidance paradigm, supporting a role for H3 receptor modulation of cholinergic function in memory acquisition. Toyota et al. Mol. Pharmacol. 62, 389-397, 2002.
Impairments in social recognition memory are apparent in AD, but may also be relevant to social cognitive impairment in schizophrenia and ADHD. Esbenshade et al. Molecular Interventions 6:77-88, 2006. Social recognition tests have been used to show that the administration of selective histaminergic agonists enhances social memory, whereas recall is disrupted by the inhibition of histamine synthesis. Prast et al. Brain Res. 734, 316-318, 1996. In particular, thioperamide as well as several other H3 receptor antagonists have been attributed with pro-cognitive effects. Id. In working memory impairments, prevalent in AD, ADHD, and schizophrenia, thioperamide reverses scopolamine-induced deficits. Barbier et al. Br. J. Pharmacol. 143, 649-661, 2004 and Fox et al. J. Pharmacol. Exp. Ther. 305, 897-908, 2003. Thioperamide, ciproxifan, and GT-2331, all H3 antagonists, are also efficacious in treating impulsivity associated with ADHD in spontaneous hypertensive rat pups. Fox et al. Behav. Brain Res. 131, 151-161, 2002.
The H3 receptor is also involved in pathological processes in the 6-OHDA (6-hydroxydopamine) lesioned rat brain, a well-characterized model of Parkinson's disease. Increased H3 receptor mRNA expression and binding may, for example, modulate GABAergic neuronal activity in dopamine-depleted striatum. Anichtchik et al., European Journal of Neuroscience, 12 (11), 3823-3832 2000.
Methamphetamine-induced hyperlocomotor activity, a behaviorally relevant model for psychosis, can be attenuated by ciproxifan in mice (Morisset et al. J. Pharmacol. Exp. Ther. 300, 621-628, 2002), as well as by the antipsychotic drug risperidone and the H3 receptor antagonist ABT-239. Fox et al. J. Pharmacol. Exp. Ther. 313, 176-190 (2005). H3 antagonists, such as thioperamide, have also been shown to reduce cumulative food consumption, weight gain and are suggested to have antidepressant activity. Esbenshade et al. supra and Perez-Garcia et al. Psychopharmacologia, 142(2) 215-220. 1999.
Accordingly, there is significant neuroanatomical, neurochemical, pharmacological and behavioral data to support the use of H3 receptor antagonists for improving cognitive performance in disease states such as neurodegeneration, cognitive impairment, Alzheimer's disease, Parkinson's disease, dementia, psychosis, depression, attention deficit disorder (ADD)/attention deficit hyperactivity disorder (ADHD), schizophrenia, obesity and sleep disorders.
Accordingly, compounds which are inhibitors of the H3 receptor find use as potential therapeutic agents in the treatment of a variety of central nervous system disorders related to or affected by the H3 receptor.